Progress in the production of integrated circuits employing large scale integration and high operating speeds has been characterized by an ever decreasing pattern rule. It was expected in the “National Technology Roadmap for Semiconductors” (Semiconductor Industry Association, 1994) that the mass scale production of 180-nm rule devices would start from 2001. Actually, the production started in 1999, two years earlier than expected. For the production of 180-nm devices, ArF (193 nm) lithography was believed promising, but KrF (248 nm) lithography has survived. Use of KrF lithography is contemplated for the mass-scale production of 150-nm devices and even 130-nm devices. The mature KrF lithography accelerates microfabrication toward an ever decreasing feature size. It is expected that the ArF and F2 (157 nm) lithography enable micropatterning to a feature size of 90 nm and 65 nm, respectively, while the candidates for further miniaturization include EB projection reduction exposure (PREVAIL, SCALPEL) and EUV using a light source of soft x-ray. In the prior art, the type of polymer for photoresist applications made a drastic change whenever the wavelength of light changed. This change is to insure a transmittance necessary for photoresist film. For example, the transition from g-line to i-line invited a change of the photosensitive material base from benzophenone to non-benzophenone type. The transition from i-line to KrF laser entailed a change from the novolac resin, which had been used for some time, to hydroxystyrene system. The transition from KrF to ArF invited a dramatic change from the double bond-bearing polymers, which are non-transmissive to this light, to cycloaliphatic polymers. To comply with F2 laser, a study is being made on cycloaliphatic polymers having fluorine atoms incorporated therein such as Teflon® for further enhanced transmittance.
For high-energy radiation with very short wavelength such as EB and x-rays, since light elements such as hydrocarbons used in resist materials have little absorption, resist materials based on polyhydroxystyrene have been under investigation.
In practice, resist materials for EB have been used in mask-writing application while the mask production technology is lately regarded problematic. Since the g-line age, reducing projection aligners have been used, with their demagnifying power being ⅕. Recently, a demagnifying power of ¼ is employed along with the enlargement of chip size and the increasing aperture of projection lens. Not only a reduction in line width as a result of progress of micropatterning, but also a reduction in line width as a result of magnifying power change pose serious problems to the mask production technology.
The exposure system for use in mask production has changed from a laser beam exposure system to an electron beam (EB) exposure system in order to increase the line width accuracy. Since definition of a finer feature size becomes possible by increasing the acceleration voltage in an electron gun in the system, the acceleration voltage has increased from 10 keV to 30 keV. An acceleration voltage of 50 keV now becomes the mainstream.
In conjunction with the increasing acceleration voltage, a lowering of resist sensitivity becomes a problem. As the acceleration voltage increases, the influence of forward scattering within the resist film is reduced so that the contrast of electron writing energy is improved, resulting in improvements in resolution and dimensional control. However, since such electrons can penetrate straight through the resist film, the sensitivity of the resist lowers. Since the mask exposure system carries out exposure by direct writing along a continuous stroke, the lowering of resist sensitivity undesirably leads to a lowering of productivity. To meet the demand for higher sensitivity, chemically amplified resist materials are then under consideration.
One problem associated with the high speed EB lithography is low throughputs. While chemically amplified resist materials greatly contributed to the achievement of higher sensitivity, the level of sensitivity achieved is still insufficient. A further increase of sensitivity is constantly required.
However, even when resist materials gain a progress toward higher sensitivity, a continuous stroke of writing with point beam takes a time of several hours in writing on a single wafer because of the increase of writing area and the complexity of writing pattern. For the projection reduction exposure using a mask, only a throughput of several wafers per hour is achieved at maximum because the cell projection as in light exposure is impossible. The low-energy e-beam proximity lithography (LEEPL) is expected to achieve a throughput of several tens of wafers per hour because of possible cell projection and increased resist sensitivity.
By virtue of the increase in accelerating voltage in mask writing and the use of chemically amplified resist material with a high contrast, a size of 500 nm positioned 125 nm above the wafer can now be written with precision through ¼-demagnifying power exposure. However, KrF survived to a device size of 130 nm, and the application of ArF is said to start from 90 nm, and F2 predicted from 65 nm. The limit of F2 photolithography is predicated to be 50 nm. At this point, the size on the mask is 200 nm. At the present, size control of 200 nm is difficult to perform only with an improvement in resolution of resist material. In the case of photolithography, a thinning of resist film greatly contributes to resolution improvement. This is because the introduction of CMP or the like has driven forward device flattening. In the case of mask manufacture, substrates are flat and the thickness of substrates (e.g., Cr, MoSi, SiO2) to be processed is predetermined by a percent light shield and phase shift control. The dry etch resistance of resist film must be improved before the film can be reduced in thickness.
It is generally believed that there is a correlation between the carbon density and the dry etching resistance of resist film. For EB writing which is not affected by absorption, resist materials based on novolac resins having better etching resistance have been developed. The novolac resins, however, are difficult to control their molecular weight and dispersity and thus deemed unsuitable for micropatterning.
Also, with respect to the extreme ultra violet exposure (EUV) at wavelength 5-20 nm which is expected as the exposure means in lithographic micropatterning to 70 nm et seq. as well as F2 exposure, the reduced absorption of carbon atoms was reported. It was found that increasing the carbon density is effective not only for improving dry etching resistance, but also for increasing the transmittance in the soft x-ray wavelength region. See N. Matsuzawa et al., Jpn. J. Appl. Phys., Vol. 38, pp. 7109-7113 (1999).
As the line width is reduced, it becomes necessary to reduce the edge roughness. It is believed that the magnitude of roughness is correlated to the molecular weight of a base polymer. Thus a number of resists based on low molecular weight polymers have been proposed for reducing the roughness. Exemplary polymers include dendritic polymers as described in J. Vac. Sci. Technol., B18(6), November/December 2000, p 3345; calixarenes as described in J. Vac. Sci. Technol., B18(6), November/December 2000, p 3424; and polyphenols as described in J. Photopolymer Sci. and Tech., Vol. 17, No. 3 (2004).
The low molecular weight compound-based resists proposed thus far include resist materials based on fullerenes. JP-A 6-19136 describes a fullerene having methacryloamide groups as the photosensitive group; JP-A 10-282649 describes a resist composition having a fullerene admixed in an ordinary resist material; JP-A 11-258796 describes a fullerene having carboxyl and tetrahydronaphthol groups; and JP-A 11-109613 describes the addition of a fullerene having malonic acid ester substituted thereon for increased solubility to ordinary chemically amplified positive or negative resist compositions.
While fullerenes are advantageous for micropatterning because of their low molecular weight and characterized by better etching resistance because of their very high carbon density, the lack of solvent solubility and alkali solubility is a problem.